Detection of TeV emission during early afterglow from poorly localized gamma-ray bursts with ground-based IACTs

Gamma-ray bursts (GRBs) are among the most luminous and rapidly evolving transients in the Universe. While space-based instruments have extended GRB observations up to energies of ∼100 GeV, the detection of very high-energy (VHE; E>100 GeV) emission from ground-based telescopes, especially during prompt and/or the early afterglow phases, remains challenging. These difficulties arise from the rapid temporal decay of GRB afterglows, strong attenuation by the extragalactic background light, observational latency, and the typical poor sky localization provided by MeV detectors such as Fermi/GBM. In this work, we investigated the prospects for detecting TeV (∼100 GeV--1 TeV) emission from poorly localized GRBs by adopting optimized follow-up strategies based on rapid tiling of large localization regions. We simulated a realistic population of GRBs informed by more than 15 years of Fermi/GBM and Swift/XRT detections and recent progress in the afterglow emission modeling. Using these simulations, we evaluated the detectability of GRB early afterglows by the next-generation imaging atmospheric Cherenkov telescopes, which are equipped with larger fields of view, as a function of latency, exposure time, and observational strategy. Our strategy can significantly enhance the detection rate; for instruments such as ASTRI and LACT, it increases by up to a factor of two compared to strategies limited to well-localized (Swift-like) events. For CTAO, our proposed approach provides up to four VHE detections per year.